One hundred ninety L-dopa-treated parkinsonian patients have been studied according to the age at onset and to age at last examination. The frequency of major mental disturbances was significantly higher in patients older than 60 years, whereas abnormal involuntary movements and on-off phenomenon were more frequent in patients with onset before age 60. The association of normal aging and of Parkinson's disease may be responsible for the prevalence of mental disease in older patients.